INBREEDING EXPERIMENTS 115 



Darwin compared self-fertilized plants with inter- 

 crossed plants in many different species. In the majority 

 of cases the self-fertilized plants were clearly inferior 

 to the crossed plants. These facts led to the belief that 

 the evil effects of inbreeding kept on accumulating until 

 eventually a plant or animal continuously reproducing in 

 that manner was doomed to extinction. His own experi- 

 mental results came far short of proving such an assump- 

 tion, however. The two plants with which inbreeding was 

 practiced the longest Ipomea and Mimulus showed 

 very little further loss of vigor after the first generation. 

 What the experiments did show, most clearly, was segre- 

 gation of the inbred stock into types differing in their 

 ability to grow as well as in minor, visible, hereditary 

 characters. In both species plants appeared which were 

 superior to other plants derived from the same source, 

 some being equal or even superior in vigor to the original 

 cross-pollinated stock. The inbred plants differed from 

 the original material most noticeably in the uniformity 

 of visible characters. Darwin's gardener stated it was not 

 necessary to label the plants, as the different lines were 

 so distinct from each other and so uniform among them- 

 selves they could easily be recognized. 



After several generations of inbreeding, Darwin 

 found it made no difference in the resulting vigor whether 

 the plants in an inbred lot were selfed or were crossed 

 among themselves. This he correctly ascribed to the fact 

 that the members of such an inbred strain had become 

 germinally alike. With less justice he attributed this 

 approach to similarity in inherited qualities to the fact 

 that the plants were grown for several generations under 



